1. Field of the Invention
The present invention relates to a fine cutting method for cutting works made of conductive metal materials such as ink-jet head parts, fine optical parts, and molds for injection molding these parts, which have a groove width or hole diameter of several .mu.m to several hundreds .mu.m, primarily ten .mu.m plus several .mu.m to several tens .mu.m, precisely at a fine dimension; and a system to be used for the method.
2. Description of the Background Art
Following the increase of instruments to which recent high technology is applied and the social demand for energy saving, the densification and miniaturization of mechanical systems have been facilitated. The demand for micro machining is very high. Integration of electronic circuits, sensors and actuators have satisfied these demands partially.
However, the demand for micro machining is not only for electronic circuit-related parts. The demand for micro machining is escalated for parts regulating fluids and physical quantities including light and magnetism, for example ink-jet heads, optical printers, microlens for photo-electromagnetic heads. Demands have been present for three-dimensional micro machining of these parts at about several .mu.m to several hundreds .mu.m, primarily ten plus several .mu.m to several tens .mu.m as the dimension for example of a groove width or a hole diameter.
Two-dimensional micro machining, as in the case of an electronic circuit, has hardly met the demand for such three-dimensional micro machining.
To satisfy the demand for such three-dimensional micro machining, attempts have been made by various methods. However, almost no method suitable for the demand for three-dimensional micro machining for example of a free face with curvature at such dimension as described above has been present conventionally. Conventional three-dimensional micro machining is described hereinbelow.
Micro machining methods are broadly divided as an additive process and an elimination process. Firstly, a conventional example of additive process is described.
Additive processes include physical deposition, chemical deposition, electroplating, melt injection, and optical formation; and it is said that physical deposition and chemical deposition are excellent methods capable of regulating lamination of an extremely thin film in an atomic unit or a molecular unit. Because these additive processes of themselves can hardly regulate precisely the crosswise spreading, however, some masking process is required. Thus, it is required to repeat a masking process together with film formation in a lamination manner, in order to apply these means to a process of forming a shape with a distribution in the thickness direction, namely a three-dimensional process.
An optical formation process is a technical process of processing a three-dimensional shape freely with no need of any photo-mask, by controlling the position of a collimated beam to a photo-curing resin for the application of the beam over a necessary part to cure only the necessary part, then immersing only a part of the photo-curing resin at a thickness to be cured, thereafter irradiating only the necessary part for curing with the beam, and repeating the steps described above to repeatedly laminate the cured layer.
Attempts to refine the optical formation process to apply the resulting process to micro machining are introduced in for example "Designing and Manufacture of Micromachine by Process using Ultraviolet Setting Resin", Machine Designing, Vol. 34, No. 15, pp. 50-55, 1990-11.
However, the additive process has a drawback in that a step difference occurs between single layers because of the three-dimensional process for lamination. To overcome the drawback, there is a method to refine the masking process or optical beam lamination or positional control instrumentally and to collimate the beam very finely for example at a single layer precision of about 0.1 .mu.m.
Nevertheless, such a method is not satisfactory as an essential three-dimensional process to form a freely curved face. The reason is that in the step difference caused by three-dimensional processing by the additive process, the variation of the shape in the thickness direction following lamination is discontinuous in proportion to the laminated thickness. Thus, in order to generate a continuously varying face by the additive process, a process for laminating a thin lamination film of about a single atomic layer at infinite times is required. This is an unrealistic process requiring an exceedingly great number of procedures, which is the reason why the additive process is not suitable for fine three-dimensional processing.
Then, the elimination process will be described.
The elimination process includes photo-etching, energy beam processing by means of a laser or electron beam, electrical discharge machining, electrolysis process, mechanical processes such as cutting, grinding, and polishing, and LIGA process described hereinafter in detail. The elimination process frequently comprises processing by placing a main device to remove materials at a point to be processed, and in this case, no essential difference in terms of processing is present in between the thickness direction and the crosswise direction vertical to the thickness direction. Therefore, it can be said that the process is suitable for three-dimensional processing.
Photo-etching generally promotes processing in a random direction, so the process requires a masking process in order to control the processing in the crosswise direction. For the photo-etching process, therefore, a work should be thin in the longitudinal direction. To apply the photo-etching process to three-dimensional processing, namely processing with a shape distribution in the thickness direction, masking processing and etching should necessarily be laminated in repetition, just as in the additive process. Thus, such photo-etching process is not suitable for fine three-dimensional processing for the same reason as for the additive process.
It has been known a process of pseudo three-dimensional processing capable of preparing a steep rise in the depth direction with no lamination procedure, by utilizing the difference in etching rate, depending on the crystallization direction. However, the process cannot freely realize a three-dimensional shape because the shape to be possibly processed is limited due to the crystallization direction.
A laser process is a process of cutting a work, for collimating coherent beams from laser oscillation into a narrow beam through an optical system such as lens and blowing out a material heated and melted locally at a high temperature. By the laser process, the beam collimation is at most about 2 to 3 .mu.m due to the limitation of optical wave length and optical systems, so that it is very difficult to process a shape below the dimension by the laser process, with the resultant precision below the value. Additionally, controls in the depth direction are difficult, so that the process is essentially grouped as a two-dimensional process. In order to carry out a process simulating a three-dimensional process, lamination is essential. Thus, the laser process is also unsuitable for fine three-dimensional processing, for the same reason as that of photo-etching.
An electron beam process is a process of processing under controls of electrons discharged from an electric field in vacuum by collimating the electrons in a beam, but the process is essentially unsuitable for fine three-dimensional processing on the basis of the same reason as that of the laser process.
A process by means of excimer laser as an ultraviolet laser, having been practically used in recent years, is a process for directly cutting a molecular bonding via photon energy with almost the same bonding energy as that of polymer such as resins. Unlike the laser process, however, the process never requires the use of heat, so that the process is an excellent process capable of realizing more fine processing. Additionally, the control of photon energy enables the control in the depth direction at some extent.
Practically, however, metals as a main material of molds and ceramics are processed thermally, with no precision expectable. Even resin processing cannot be implemented freely as in the case with tools and devices, and therefore, the current process by means of an excimer laser still remains at a status of two-dimensional process.
E. W. Becker et al. disclose a LIGA process in "Fabrication of Microstructures with High Aspect Ratios and Great Structural Heights by Synchrotron Radiation Lithography, Galvanoforming, and Plastic Molding (LIGA process)" Microelectronic Engineering, vol. 4, pp. 35-36, 1986 (acronym of Lithografie, Galvanoformung, und Abformung, in German, meaning lithography, electric plating and molding, respectively), capable of overcoming such a practical limitation of photo-etching that photo-etching process can implement only two-dimensional process.
This process is an excellent process, which can achieve the fabrication of a fine structure with such an extremely high aspect ratio as a width of about 1 to 10 .mu.m and a height of about several hundreds .mu.m, by exposing an extremely thick photoresist to light by using a highly linear X-ray at a high power density of a giga eV grade from a synchrotron. The process can attain the highest fineness, for utilities with no need of any shape distribution in the thickness direction.
Because the LIGA process is characterized by the high aspect ratio owing to the linearity of X-ray, however, the LIGA process cannot yield a shape distribution such as free face with curvature in the thickness direction. Thus, the process is not suitable for fine three-dimensional processing.
In addition to those described above, the elimination process includes electrical discharge machining, electrolysis processing, ultrasonic processing and mechanical processing, by means of tools and devices. At these processes, tools are positioned three dimensionally, whereby the position to be processed can be controlled. Thus, it is said that these processes are essentially suitable for three-dimensional processing.
As described in "Microprocessing Technique, Basic Lecture Series, Micro Electro Discharge Machining", issued by Nikkan Kogyo Shinbun.Ltd., the electrical discharge machining can control the energy of generated discharge pulses below an energy as extremely low as 1 .mu.joule, owing to the instrumental technique of a discharge electric source. As described in "Research of Micro Electro Discharge, Vol. 3, Processing of Micromachine", Journal of the Japan society of Electrical-Machining Engineers, Vol. 28, No. 59, p.1-p.10, three-dimensional positioning of an extremely small electrode under controls can establish fine three-dimensional processing.
Even under any control of discharge conditions, due to the discharge phenomenon used as a processing principle of electrical discharge machining, wherein positive and negative charged particles are involved, the process cannot prevent the damage of a tool electrode with a small number of charged particles, so that the consumption of the electrode can scarcely be reduced to zero. As described in the "Research of Micro Electro Discharge Machining", 3.sup.rd Report, Processing of Micromachines, the wear of the electrode deteriorates the dimensional precision of a work, which causes difficulty in three-dimensional free processing.
Additionally, an example of fine process through electro discharge machining using a fine wire is described in "Research of Fine Wire Electro Discharge Machining--Processing Technique using Wire Electrode of 10.mu.m Diameter", Electric Processing Technique, Vol. 17, No. 57, p.13-p.18, 1993. The process is excellent with practically negligible electrode consumption, owing to continuous feeding of a fresh electrode. However, the shape to be processed is for example just a shape sawed with a thread saw, so that the process cannot fabricate a shape with a recessed face.
It is also described in Precision Machining of Micro Spindles, 1.sup.st Report, Development of Wire Electrodischarge Grinding Method", Journal of Japan Society of Electro-Machining Engineers, Vol. 25, No. 48, p.14-p.23, a process capable of processing a fine shaft and a recessed face by folding a wire by means of a guide wire and using just the folding point as a processing point. The process is called as WEDG (Wire Electrical Discharge Grinding) process. However, the radius of the curvature of the wire and wire guide to be used at the process correspond to the diameter of a tool, which limits the size thereof to about several mm. Thus, the process is applicable to fine three-dimensional processing with much difficulty.
There has been known a process of fabricating a work, by using a similar electrode, for immersing the work piece in an electrolytic solution followed by applying of a voltage, and carrying out the fabrication by utilizing the electrolysis and dissolution. As described in "Overview of Electrolytic Machining", Metal Surface Technique, Vol. 31, No. 1, p.2-11, the process is advantageous in that no electrode wear occurs, unlike electro discharge machining. However, electrolysis phenomenon is not sensitive to the distance from an electrode, as is observed in electro discharge machining, so that the gap between the electrode and the work piece is large while the precision is at most .+-.0.03 mm, approximately. Thus, the electrolytic machining is limited to the processing of a dimension of about 0.1 mm, and the processing of a fine three-dimensional shape in unit .mu.m is extremely difficult.
Additionally, "Ultrasonic Piercing", Machinery, Vol. 27, No. 27, p.1011-1020, 1964, describes a process for giving ultrasonic vibration to a tool and placing a abrasive grain on the tip of the tool and crushing a work piece through the vibration of the tool tip in a fine fashion. The ultrasonic process is effective for rigid and fragile materials to be processed, such as ceramics and glass. The precision of ultrasonic process depends on the diameter of the abrasive grain, and the tool itself is then worn. Thus, the precision of the process can hardly be below .+-.0.01 mm, currently.
Meanwhile, mechanical process carries out processing by physically putting a tool and a work piece in contact together to locally deform plastically the work piece and then separating and drawing out the material. Fine processes by using the principle of such mechanical process include a three-dimensional process by instrumentally positioning a tool under controls by using so-called general mechanical process for cutting by means of a cemented carbide tool and a diamond tool and a process using a different process from traditional mechanical processing technique.
As described for example in "Research of Vibration Processing System in Fine Field", Vol. 2, Proceedings of Spring-term Federation of Japan Society for Precision Engineering in 1995, p. 533-p.534, a process for cutting and processing an extremely micro region through ultrasonic vibration of a fine needle of single crystal diamond is known as a process employing a mechanism different from those by conventional mechanical processes. As described for example in "Mechanical Process in 1 nm Depth Unit of Mica under Atomic Force Microscope", Proceedings of Autumn-term Federation of Japan Society for Precision Engineering in 1994, p. 723-p.724, the removal and processing of mica in molecular unit can be attained via the frictional force from a probe of an atomic force microscope. Furthermore, a mechanical process by using an STM tungsten probe is described in "Mechanical Fine Processing by STM", Proceedings of Spring-term Federation of Japan Society for Precision Engineering in 1994, p. 5-p.6.
These processes can reduce the processing unit to a unit of atom or molecule as the essential requirement for fine processing, excellently.
However, a steric structure up to about several-.mu.m cube is an area to be processed by these processes, so such processable area is too small, compared with an area of about several .mu.m to several hundreds .mu.m, primarily ten plus several .mu.m to several tens .mu.m, to be required for an ink-jet printer and microlens array. Therefore, the processes require a processing time of several tens of minutes to process a cube of several .mu.m. Hence, the processing rate of these processes is currently too slow.
In order to fabricate a fine shape by using conventional cutting tools, it is essential that the cutting tools themselves should be small. The most serious problems involved in the fine cutting of the cutting tools include the wear thereof due to relative friction and the deformation and breakage of the cutting tools due to counter force. In order to overcome these problems, therefore, various attempts have been made conventionally.
Conventional technique for the purpose of fine cutting by mechanical processing is described below. These attempts are grouped into those mainly for the improvement of the cutting tools themselves, those mainly for the improvement of motion accuracy for processing, those mainly for the improvement of the machinability property of a work piece, and those mainly for the improvement of the environment such as processing atmosphere.
Firstly, a technique for the improvement of the cutting tools themselves mainly includes devices to prevent the wear and break of the cutting tools. Examples of such devices include a preventive device of the breakage of a cutting tool, by preparing the tool in a double structure made of a cemented carbide alloy of a highly rigid composition at the center of the tool and a cemented carbide alloy of a hard composition at a surface layer of the tool as described in Japanese Published Unexamined Patent Application No. 61-57123.
Additionally, a great number of devices for the improvement of the machinability property and wear resistance of tools have been attempted. Examples thereof include a coating process of a diamond film onto the surface of a cemented carbide tool, as described in Japanese Published Unexamined Patent Application No. Hei 1-201476.
Then, a conventional technique mainly for the improvement of the motion elements in processing is described below.
As discussed in "Drill Grinder with Fine Diameter and Performance of Centering Device", Sugawara et al., Proceedings of Autumn-term Federation of Japan Society for Precision Engineering in 1983, p.41, the centering of the rotation axis of a tool, the positioning of the rotation axis along the feeding direction of the tool, and the feeding straightness thereof are important for preventing the break and wear of tools such as microdrill and micro end mill. A device of such centering system and a device for reducing tool runout are described therein.
However, all of these devices are concerning how to carry out centering and chucking of a drill separately prepared. From the respect of general tool vibration, vibration at a level of 1 .mu.m still remains, which is a serious problem for a fine tool, although such vibration is not so large.
As a technique to overcome such problem, it is described in "Application of WEDG onto Microdrill and End mill", Proceedings of Spring-term Federation of Japan Society for Precision Engineering in 1989, p.1091-p.1092 or "Development of Multi-purpose Fine Processing Machine for Concurrent Use for Discharge and Cutting", Proceedings of the 1-th National Conference of the Japan Society of Electrical-Machining Engineers, p. 111-114, 1991, a machining process for drill processing and end mill processing, comprising processing a fine microdrill or a fine end mill by using the WEDG process, and effecting the drill or end mill processing on the processed tool by using the tool itself.
Compared with conventional processes for mounting a drill or an end mill manufactured by a tool manufacturer onto a tool machine, the process can reduce the tool runout to zero, which vibration is due to the shift of the rotation center during the manufacture of a tool from the rotation center during the process by means of the tool, and therefore, the process is very excellent.
Then, those mainly for the improvement of the machinability property of a work piece are described below. Compared with the process for the purpose of improving tools, only a few examples thereof are illustrated. For example, Japanese Published Unexamined Patent Application No. Hei 5-9701 describes an example of thermal treatment of titanium. In this example, an oxide scale layer is prepared on the surface of titanium by heating titanium at 450.degree. C. or more, and because of the fragile and chemically inactive properties of the resulting scale layer, the resulting titanium material can get good processability, whereby the wear of the tool is reduced. This is a representative attempt to control the properties of a work piece.
Japanese Published Unexamined Patent Application No. Sho 60-100601 discloses a technique for improving the cutting property of an iron sintered material, comprising processing the material in steam thereby producing an oxide film thereof. Additionally, Japanese Published Unexamined Patent Application No. Sho 63-105982 discloses a process of realizing an easier cutting process, for modifying the mechanical performance of ceramics through the chemical effect of wet etching and the effect of promoting chemical reaction with a laser.
As a technique to prepare a cutting atmosphere, Japanese Published Unexamined Patent Application No. Sho 63-102844 discloses a method for preventing oxidation of a tool thereby preventing the wear of the tool, comprising covering the cutting part thereof with a gas from which oxygen is removed.
Conventional techniques insofar described for the purpose of fine processing by machinery processes have securely improved fine mechanical processing, but the techniques are not satisfactory means to essentially overcome the wear or break of an extremely small tool. This will be described, through the calculation of the break conditions during end mill processing. The relation among them is described for example in "Deka-Ban Technical Books, All about End mill", issued Taiga Shuppan, K. K.
At the end mill process, tools are frequently required to be transferred in the vertical direction to the rotation axis, so that a bending moment works on the tools; as the tool diameter is smaller, the possibility of break, namely break and damage, due to the bending moment, is higher. This is represented by the following formula; EQU R=a.times.V.sup.p
wherein "R" represents cutting resistance (N/mm.sup.3); "a" and "p" are material coefficients; and V is a volume removed per unit time.
This formula is discussed for example in the case of end milling. Below 20 .mu.m of a tool diameter, the tool breaks, so that the resulting area is fallen into a state such that cutting of the area is nearly impossible. Hence, the fine processing by means of fine end mill depends on the extent how much the bending strength can be suppressed.
As described in Japanese Published Unexamined Patent Application No. Hei 1-201476, alternatively, the coating with a superficial hard film on a tool of a cemented carbide is effective for preventing the wear of the tool when a to-be-processed material with a higher hardness is processed, but the coating is not effective for the break during a process by means of fine end milling.
The double structure of a drill is effective for elevating the bending moment value as a break provision, but the fabrication of a drill of a double structure and with a dimension below 50 .mu.m of itself is difficult.
It is believed that the use of the WEDG process described above is an essential element in order to overcome the problem of rotation vibration as one of the most significant factors of the breakage of microdrill. However, the process can prevent the application of a non-uniform and abnormal force onto a tool because of the vibration and rotation of the tool, but the process has no effect of modifying the break conditions or the bending moment value of itself. Thus, the process is insufficient, from such respect.
On the basis of the same reason, the device to adjust cutting atmosphere is not effective from the respect of break prevention, but the device rather focuses attention toward the prevention of wear. In other words, such an approach to improve the mechanical properties of tools can hardly carry out three-dimensional processing of about ten and several .mu.m to about several tens .mu.m.
It can be said that the device to impart a property to be readily cut to a work piece is highly effective for fine cutting and the device has a possibility of processing at an objective dimension. However, the processes described as the prior art have the following drawbacks.
More specifically, the process as described in Japanese Published Unexamined Patent Application No. Hei 5-9701, for heating titanium at 450.degree. C. or more to generate an oxide scale layer on the surface, induces the preparation of the oxide scale layer on a wide area of the surface of a work piece because of the application of heat. Thus, it is impossible to limit the processing only to an intended area.
Additionally, because the superficial oxide scale layer is inappropriate as a mold, the layer should necessarily be processed, but the entire microfabricated complex area with recesses and protrusions can be difficult to remove. Therefore, the method described in the Japanese Published Unexamined Patent Application is not appropriate as a technique to prepare a microfabricated mold.
Those described above are also true with a process of improving the property of an iron sintered material to be cut for processing the material in steam thereby preparing an oxide film, as described in the Japanese Published Unexamined Patent Application No. Sho 60-100601, and the process in the publication is inappropriate as a technique for preparing microfabricated molds.
A process of modifying a cutting process of ceramics as readily processable by modifying the mechanical properties of ceramics through the mechanical effect due to grinding, the chemical effect due to wet etching, and the effect of propagating chemical reaction with a laser, as described in Japanese Published Unexamined Patent Application No. Sho 63-105982, wherein the intensity of laser irradiated has a distribution, hardly controls the positioning of a part where the mechanical properties should be controlled at a precision in the order of 0.1 .mu.m which is required for three-dimensional processing at about ten plus several .mu.m to several tens .mu.m, as the object of the present invention. Thus, the process is difficult to apply to fine cutting. Furthermore, ceramics is a fragile material and is inappropriate for a mold from the respect of durability, disadvantageously.
A means for regulating the mechanical performance of a limited region of a work piece, including a metal material appropriate for molds, for example, aluminum, titanium, iron, nickel and cobalt, has been demanded, and the limited region should be at a level with a possibility of fine cutting.